1. Field of the Invention
The present invention relates to a vertical injection molding machine for molding a product by clamping and releasing molds in the vertical direction, and more specifically, relates to a vertical injection molding machine having a turntable to which molds are mounted.
2. Description of the Related Art
Heretofore, a so-called two-station type vertical injection molding machine has been used, wherein two lower molds mounted on a turntable are moved to positions capable of being clamped with a single upper mold by the revolving motion of the turntable, and the molds are clamped in the vertical direction. According to this so-called two-station type vertical injection molding machine in which a plurality of lower molds are clamped with an upper mold, a mold product having a metallic insert component integrally molded with resin is manufactured by assembling a metallic component for insert-molding to the lower mold positioned on the opposite side from the lower mold being clamped with the upper mold during a mold clamping process, and after completing assembling, revolving the turntable to a predetermined position so as to clamp the lower mold having the metallic component assembled thereto with the upper mold.
In order to produce such molded products in which metallic insert components are integrally molded with resin, it is necessary to assemble the insert component in advance to one of the molds before clamping the molds, so the lower molds being revolved together with the revolving movement of the turntable must be stopped accurately without displacement at predetermined positions capable of being clamped with the upper mold. FIG. 10 is an explanatory view showing the relevant portion of such two-station type vertical injection molding machine according to the prior art described above, which will now be described in detail with reference to the drawing. According to the illustrated vertical injection molding machine, a turntable 102 on which two lower molds 100 and 101 are mounted is revolved in both clockwise and counterclockwise directions by a servomotor 104 via a timing belt 103. A stopper pin 105 is integrally formed to a lower surface of the turntable 102 on which the lower molds 100 and 101 are mounted that are clamped with the upper mold not shown. When the turntable 102 is revolved by activating the servomotor 104, the stopper pin 105 that moves together with the revolution of the turntable 102 comes in contact with the stopper 106, the state of which is illustrated in FIG. 10, and the lower mold is held at a position capable of being clamped with the upper mold by the stopper pin 105 being pressed against the stopper 106 with predetermined torque until the mold clamping process in which the lower mold 100 and the upper mold are clamped is completed. Japanese Patent Application Laid-Open Publication No. 2002-172646 (patent document 1) discloses a prior art turntable of an injection molding machine having a structure related to the art described above.
However, since according to the vertical injection molding machine of FIG. 10, the stopper pin is repeatedly pressed against the stopper for each molding process by the revolution of the turntable in both directions, the contact portions of the stopper pin and the stopper are both worn. When the vertical injection molding machine is operated for a long period of time, the components such as the stopper pin used for positioning are worn away, making it difficult to stop the lower molds at predetermined positions. Moreover, if the vertical injection molding machine is continuously operated in the state in which the contact portions are worn away, the molds subjected to clamping are displaced from the predetermined positions, and in some cases, the molds may even be damaged during clamping. Further, since it is necessary to replace worn components such as the stopper pin and the stopper periodically, the replacement of worn components requires much time and work, and the operation of the molding machine must be interrupted each time the component is to be replaced, so that the manufacturing costs of molded products are increased in vertical injection molding machines producing molded products having insert components made of metal or the like integrally molded with resin.
Further, when the stopper pin collides against the stopper, not only the stopper pin and the stopper but also the drive system of the turntable may be damaged. Therefore, it becomes necessary to stop the turntable without fail immediately before the stopper pin contacts the stopper, which is enabled by taking a large reduction distance before the stop position. However, by increasing the reduction distance, the time required from the starting of the revolution of the turntable to the stopping thereof is increased, and as a result, the overall molding cycle is elongated. Further, as in the case with the two-station type vertical injection molding machine having two molds mounted on the turntable as shown in FIG. 10, if the two molds on the turntable are revolved in reciprocating motion for 180 degrees at a time with the revolution of the turntable, the lower molds can be stopped at predetermined positions by having a stopper pin contact a stopper fixed to a base or the like, but in a three-station machine or a four-station machine, the turntable must be revolved for 120 degrees at a time in the former machine and 90 degrees at a time in the latter machine in the same revolving direction, which requires a complex mechanism or control system to engage or release the stoppers correspondingly, not only deteriorating reliability but also increasing costs.
Furthermore, according to the example illustrated in FIG. 10, the turntable is stopped by having the stopper pin press against the stopper, but if such structure is not adopted and the revolving turntable is stopped only by the control of the servomotor being the drive source of the turntable, the turntable not only fails to stop within a predetermined range but also the stop position is displaced greatly along with the passing of turntable operation time. Therefore, the inventors of the present invention have conducted various tests, and arrived at the following conclusions regarding the mechanism of dispersion of the stop position of the turntable.
According to test results, when the turntable revolution is reduced and stopped, if the rotational resistance of the turntable is smaller than the rotational inertia thereof, the turntable continues to revolve by inertia resisting against the rotational resistance even when the servomotor is at reduction, so the turntable overruns the predetermined stop position and stops at an overrun position even when the servomotor performs a turntable positioning control. On the other hand, if the rotational resistance of the turntable becomes greater than the rotational inertia, the force acts in the opposite direction, and as a result, the turntable tends to stop before the predetermined stop position. Thus, the present inventors have discovered that the rotational resistance of the turntable is influenced by the sliding conditions of the sliding portions of the turntable or the shaft bearing portion, and especially influenced greatly by the sliding conditions of the sliding portions of the turntable having a large sliding resistance. When the sliding portions are fed with sufficient amount of lubricant and the sliding conditions thereof are good, the rotational resistance is small, but when the sliding portions lack lubricant and the sliding conditions thereof are poor, the rotational resistance is increased correspondingly. Therefore, the dispersion in the stop position of the turntable is recognized to be caused mainly by the sliding resistance being varied greatly, which is caused by the change in sliding conditions of the sliding portions of the turntable.
Furthermore, in order to detect the stop position of the turntable mentioned above, the number of revolutions of the servomotor acting as the drive source for revolving the turntable is output to an encoder, and via the count of pulses output from the encoder, the stop position of the turntable revolving with the servomotor can be detected, and thus, the position of the turntable, or angle, can be detected accurately. Now, if four molds are mounted on the turntable so as to realize a four-station machine that revolves 90 degrees at a time, and the turntable is to be revolved continuously in the same direction, by adopting the prior art stopper method, the accurate position of the turntable or angle thereof can be detected by resetting the encoder pulses when the turntable is revolved for 90 degrees at a time and comes into contact with the stopper. However, in a device that does not utilize the stopper, the number of pulses of the encoder will increase continuously and finally exceed a maximum number of pulses countable by the encoder if the turntable is continuously revolved in the same direction, and the accurate position of the turntable, that is, the angle of the turntable, can no longer be detected. In order to prevent this problem from occurring, it is necessary to reset the count before the number of pulses exceeds the maximum number of pulses countable by the encoder, but it is difficult to detect the position or angle of the turntable accurately without a stopper and to reset the count.